Thermal printing materials generally comprise a substrate and a thermal printing layer formed thereon composed mainly of an electron-donative, colorless dye precursor and an electron-attractive developer. When they are heated by means of a thermal head, a thermal pen, laser beam, or the like, the colorless dye precursor reacts with the developer in a moment to form a printed image. They are disclosed in Japanese Patent Examined Publication Nos. 43-4160 and 45-14039, etc. Such thermal printing materials are advantageous in that they permit printing by means of a relatively simple apparatus, are easy to maintain, and do not cause production of a noise, and they are used in various fields, for example, in recorders for measurements, facsimiles, printers, terminals of computers, labels, and vending machines for tickets and the like.
Such thermal printing materials using an electron-donative, colorless dye precursor and an electron-attractive developer have various excellent characteristics, for example, they have a good appearance, are good to the touch, show a high depth of developed color, and can give various hues of developed color. But they are disadvantageous in that they are poor in print storage properties, for example, a thermally color-developed portion (a printed image portion) is lost owing to plasticizers, additives, etc. contained in plastics such as poly(vinyl chloride) when brought into contact with plastics, or is easily lost when brought into contact with chemicals contained in foods or cosmetics, or is easily faded by exposure to the sunlight for a short period of time. In the existing circumstances, because of this disadvantage, they are limited in uses to a considerable degree and are eagerly desired to be improved.
Further, in recent years, a high-speed printer capable of printing in a short time has been developed, and there has come to be required a highly sensitive thermal printing material suitable therefor which is excellent in response to heat and can give a sufficient developed color image even at a low energy.
As thermal printing materials in which two components react with each other on heating to give a printed image good in storage properties, those in which the two components are an imino compound and an isocyanate compound are disclosed, for example, in Japanese Patent Unexamined Publication Nos. 58-38733, 58-54085, 58-104959, 58-149388, 59-115887 and 59-115888, the specification of U.S. Pat. No. 4,521,793.